Allowing users to retrieve ciphered VM password

When retrieving a VM password by using the python-novaclient (for
instance by using the `nova get-password` command), one has to provide
its private key, for the client to be able to decrypt and display the
VM password.

On the other hand, one may prefer using python-novaclient only to
retrieve the ciphered password, and then decrypt it by himself (or by
using another piece of software). For instance, a user managing its
VMs through the Horizon dashboard may prefer to decrypt the password
locally on its computer, rather than sending his private key over the
network.

This blueprint proposes to allow users to retrieve the ciphered
password through python-novaclient, when not specifying any private
key. The client's behavior would remain unchanged when the key is
provided.